1. Field of the Invention
The invention relates to the combination of double walled, thermally insulated pipes or conduits with skin effect heat tracing pipes.
2. Description of the Related Art
As it is well known, when delivering products such as crude oil having a solidification temperature higher than the ambient temperature and which must therefore be kept at a temperature higher than ambient during delivery and while located in the delivery pipes or conduits, it is necessary to thermally insulate the delivery conduit to help control any heat loss. However, it is also desirable that the insulating properties remain stable for a long period of time. This is particularly difficult in subsea environments or during handling of the conduit.
To this end, a thermally insulated, double wall conduit with special joint connectors to ease installation was developed. This conduit and joint connectors are described in U.K. Patent No. 2,161,565, which is hereby incorporated by reference, and which are called the Double Pipe Insulated System (DPIS) available from Snamprogetti, Milan, Italy.
While the DPIS conduit alone is adequate in many situations, there are also cases where heat must be provided to the material being delivered to keep it sufficiently fluid to allow easy delivery. This heat generation is done in conventional conduit systems using some form of heat tracing. Alternatives include elongated tape heaters, but in many pipeline environments the use of skin effect heat tracing pipes is preferred. In a skin effect heat tracing pipe, the inner surface of a ferromagnetic pipe is electrically energized. An insulated, non-ferromagnetic return conductor is used to complete the circuit. Because of the characteristics of skin effect:, the inner surface of the pipe is energized and heats up but the outer surface is at ground potential. Further examples of skin effect heating are provided in U.S. Pat. Nos. 3,293,407 and 4,645,906, both of which are hereby incorporated by reference.
Conventionally the skin effect pipes are simply in contact with the outer surface of the delivery conduit and thermal conduction is used to transfer the heat from the skin effect pipe to the delivery conduit to the material. However, this is unacceptable when using conduit such as DPIS, because the intermediate insulation limits the transfer of heat into the central process conduit, just as it limits heat transfer out of the process conduit. But as stated, in some cases it is desirable to provide heat to the process conduit to heat the material being delivered. As noted, the dual wall construction of DPIS conduit interferes with the conventional techniques, so that in these cases DPIS conduit may not be used. Therefore it is desirable to be able to heat trace DPIS conduit, particularly using skin effect pipes.